Implantable cardiac stimulation devices include devices such as cardiac pacemakers, cardioverters, and/or defibrillators. These devices stimulate the heart to maintain a patient's cardiac activity to meet metabolic needs. Originally, pacemakers stimulated the heart at a fixed rate. This did not provide an adequate heart rate based upon changes in the physiologic and metabolic needs of the patient.
It has been recognized that it is necessary to monitor physiologic and metabolic parameters to change the stimulation rate as indicated by the activity and stress levels of the patient. It has also been recognized that there is a need to monitor multiple sensors to determine the indicated stimulation rate.
These sensors can include minute ventilation (also known as minute volume), paced depolarization integral (PDI) (also known as ventricular gradient), QT interval, activity level, activity variance, temperature, oxygen saturation, the inclination of the patient's body, pre-ejection period (PEP), etc.
This invention is drawn towards the sensors which require a baseline measurement, typically at rest, and preferably not during sleep. Such sensors include minute ventilation (also known as minute volume), paced depolarization integral (PDI), QT interval, and pre-ejection period (PEP), oxygen saturation, temperature, among others. Such sensors tend to rely on the integrity of the stimulation lead for proper operation. A few of these sensors also have a tendency to drift out of calibration due to the patient's changing exercise needs, medications, etc.
As a result, a method of calibration is useful for any sensor which is dependent upon lead integrity for proper operation and/or which requires periodic re-calibration. It is also desirable to determine a sleep value, below the alert resting state, to enable patients to achieve a lower pacing rate while sleeping.
The minute ventilation of a patient, for example, is based on tidal volume and respiration rate, which may be detected by measuring the amplitude and rate of a patient's respiration impedance signal. The measurement of minute ventilation is well known (see, for example, U.S. Pat. No. 5,562,712, issued Oct. 8, 1996 to Steinhaus et al., entitled "Minute Volume Rate-Responsive Pacemaker using Dual Unipolar Leads"). Briefly, the impedance signal may be obtained through the use of a controller applying a measuring current between a first electrode and a reference point on the pacemaker, typically the housing, sometimes referred to as the case electrode. The impedance can then be measured, typically, between a second electrode and the reference point. This impedance measurement of the patient varies as a function of the patient's pleural pressure, and therefor the impedance represents the patient's minute ventilation.
U.S. Pat. No. 5,707,398, issued Jan. 13, 1998 to Lu, entitled "Automatic Determination of Optimum Electrode Configuration for a Cardiac Pacemaker" sets forth a stimulation system which recognizes the need to monitor the lead impedance for changes. This system monitors the response of each electrode and then chooses the optimal electrode to monitor and measure the minute ventilation. However, this system does not address the need to test the lead impedance as a condition prior to calibrating the baseline value. Rather, it automatically selects the pair which provides optimal performance.
To calibrate the baseline value of a device employing minute ventilation, the controller must be operating under known conditions, that is, the patient must be at rest and not sleeping. Many sensors can be used to indicate when the patient is at rest and/or in a sleep state; such sensor signals include the activity level, the activity variance, and possibly the inclination of the patient. See, for example, U.S. Pat. No. 5,626,622, issued May 6, 1997 to Cooper, entitled "Dual Sensor Rate-Responsive Pacemaker", which shows the use of an activity sensor to determine the activity level of the patient. See, for example, U.S. Pat. No. 5,476,483, to Bornzin et al., entitled "System and Method for Modulating the Base Rate During Sleep for a Rate-responsive Cardiac Pacemaker", which shows the use of activity variance to determine if the patient is at rest or sleeping.
Accordingly, it is desirable to develop an implantable cardiac stimulation device which can perform an enhanced calibration of its baseline level upon implantation (i.e., by performing a self-test of the lead system and verifying that the patient is at a suitable resting state) and to automatically and periodically recalibrate the baseline at appropriate intervals to ensure the correct stimulation rate may be determined for the patient.